1. Field of the Invention
The invention relates to liquid crystal displays (LCDs) and, more particularly, to improving the viewing angle characteristics of liquid crystal displays.
2. Description of the Related Art
Most modern liquid crystal display panels suffer from poor viewing angle characteristics (color shift and level reversal, as a function of viewing angle) over a range of subpixel intensity values between the bright and dark states. Of the various liquid crystal modes used in these displays, the most commonly used is the Twisted Nematic mode (TN mode), which has poorer viewing angle characteristics than other modes. Typically, a normally white mode is used, so that the fully bright state corresponds to a low applied voltage and the fully dark state corresponds to a high applied voltage. The display picture elements are commonly referred to as pixels, where each pixel usually consists of a group of three subpixels, namely red, green, and blue subpixels. Typical LCDs have a stripe pixel geometry, where the pixels are square in shape, and where all subpixels are shaped as vertical stripes with the height of a full pixel and width of one third of a full pixel. For the normally white mode, using 8-bit drive per color, the highest applied voltage corresponds to an intensity value of zero, and the lowest applied voltage corresponds to an intensity value of 255. Intensity values are also referred to as digital pixel levels, or digital to analog conversion values (DAC values).
The poor viewing angle characteristics result from the variation in optical transmission at different angles as voltage is applied across the liquid crystal cell gap. At a viewing angle of normal incidence to the surface of the display, the luminance increases with digital pixel level, roughly following a power law, generally referred to as a gamma curve. FIG. 1 is an idealized gamma curve illustrating the relationship between luminance and digital pixel level at normal incidence. At viewing angles away from normal incidence, the gamma curve becomes distorted. For a given digital pixel level, the luminance varies strongly with viewing angle. FIG. 2 shows the general trend of relative luminance variation over all viewing angles as a function of the digital pixel level. The variation in luminance has a non-monotonic dependence on pixel level, with the largest variation occurring over a range of pixel levels somewhere between the dark state and bright state.
U.S. Pat. No. 5,847,688 to Ohi et al. describes a technique that provides a new set of analog reference voltages to the data drivers every other frame. This requires additional, specialized circuitry to be added to the drive electronics for the panel. To work well, the method requires reference voltages for different gamma curves to be switched every two or more frames. This is necessary to provide both positive and negative voltages sequentially to the pixel. If the frame rate is 60 Hz, the switching rate of the gamma curve would be 30 Hz or less. If the modulation in luminance between the two gamma curves is large enough, as required to improve the viewing angle characteristics, then flicker will occur. Human visual sensitivity to flicker peaks at about 10 Hz, and the sensitivity at 30 Hz is quite large. Alternatively, if the liquid crystal response speed is not fast enough to fully respond within two frame times, then the liquid crystal director will maintain an average position within the cell structure, and the luminance will not vary with time. The resulting luminance value will be the average of the two gamma curves, and no improvement in viewing angle characteristics will occur.
U.S. Pat. No. 5,489,917 to Ikezaki et al. describes a technique whereby the reference voltage set is altered from the usual condition in that the lowest reference voltages are increased to suppress level reversal. For TN-mode LCDs with the usual rubbing and polarizer configuration, this method improves the viewing angle characteristics in the upward direction (downward-looking) only. The level reversal condition is much stronger in the downward direction (upward-looking), so this method does not address the most noticeable deficiency in the vertical viewing angle characteristics. The method requires that the total range of reference voltages be decreased, which significantly reduces the dynamic range and contrast ratio of the panel.
G. S. Fawcett and G. F. Schrack in xe2x80x9cHalftoning Techniques Using Error Correction,xe2x80x9d Proceedings of the SID, Vol. 27/4, pp. 305-8 (1986), describes general algorithms for producing halftone images on any device, display, or printer which has limited grayscale capability. U.S. Pat. No. 5,254,982 to Feigenblatt et al. describes a halftone method with time-varying phase shift which was intended for LCDs with relatively few intensity grayscale values. The goal of both Fawcett et al. and Feigenblatt et al. is to produce nearly continuous-tone images with devices which have limited grayscale capability. The present invention is intended for use with LCDs with full grayscale capability, and takes full advantage of this capability. Finally, the techniques of Fawcett et al. and Feigenblatt et al. do not provide a method to improve the viewing angle characteristics with the halftone process.
In work done by both Honeywell and Hosiden Corporation, a split pixel structure has been used to increase the acceptable viewing angle range of TN-mode TFTLCDs. This work was described by Sarma et al. in xe2x80x9cActive-Matrix LCDs Using Gray-Scale in Halftone Methods,xe2x80x9d SID Digest, pp. 148-150 (1989); Sarma et al. in xe2x80x9cA Wide-Viewing-Angle 5-in.-Diagonal AMLCD Using Halftone Grayscale,xe2x80x9d SID Digest, pp. 555-557 (1991); Sunata et al. in xe2x80x9cA Wide-Viewing-Angle 10-Inch-Diagonal Full-Color Active Matrix LCD Using a Halftone-Grayscale Method,xe2x80x9d Int. Display Res. Conf. Record, pp. 255-257 (1991); Ugai et al. in xe2x80x9cDeployment of Wide-Viewing-Angle TFT-LCDs Using Halftone Gray-Scale Method,xe2x80x9d Electronics and Communications in Japan, Pt. 2, Vol. 80, No. 5, pp. 89-98 (1997). A summary of this work is also given in U.S. Pat. No. 5,847,688 to Ohi et al. In this technique, each subpixel is divided into two smaller split subpixels. An additional storage capacitor is utilized in combination with different load capacitances of the two split subpixels to provide a different pixel voltage to the two split subpixels. In this way, for a given subpixel voltage applied to the combination of two split subpixels, the transmission of the split subpixels is not the same. This technique is described by the authors as a xe2x80x9chalftone gray-scale method.xe2x80x9d The method is halftone in the sense that one split subpixel is brighter than the other. Because the ratio of voltages applied to the split subpixels tracks as the ratio of the capacitances, the ratio of voltages will be approximately the same for all subpixel levels. For a given subpixel voltage, and different smaller-subpixel voltages, the transmission and viewing angle characteristics of the two small subpixels are not the same. By mixing together the light from the two smaller subpixels, the viewing angle characteristics are also mixed and improved as compared to a single subpixel. A major disadvantage of this approach is that a special subpixel structure is required within the array on the glass panel. To date, this technology has been successfully applied in aircraft cabin entertainment displays, containing subpixels as small as 159 by 477 microns. As the pixel area is decreased, the additional storage capacitance and split pixel structure become increasingly difficult to implement. This limits the extent to which this approach can be applied to computer information displays, in which both a large number and large density of pixels is required. For example, a display with 200 pixels per inch requires subpixel dimensions of approximately 42xc3x97126 microns.
Ogura, et al., in xe2x80x9cA Wide-Viewing-Angle Gray-Scale TFT-LCD Using Additive Gray-Level Mixture Driving,xe2x80x9d SID Digest, pp. 593-596 (1992), describe a technique for improving the viewing angle characteristics of TFTLCDs by using additive gray-level mixture driving. In that work, pixels in odd columns are supplied with pixel voltages different from pixels in even columns. The voltage difference between columns is held at a constant value, slightly less than the threshold voltage of the liquid crystal material. The technique requires a dual-bank data driver arrangement, in which alternate columns are connected to data driver chips above and below the array. Furthermore, the top and bottom banks of data driver chips must have different sets of reference voltages supplied to them. This approach was applied to a normally-white twisted-nematic o-mode LCD. It was found that the horizontal viewing range was increased by about 10 degrees. This paper contains the understanding that pairs of pixel columns can be combined to improve the viewing angle characteristics. One deficiency of the technique is that a special, on-glass configuration is required, namely a dual-bank configuration. The control electronics must also be modified to provide an extra set of reference voltages. Another problem is that a constant offset between column pixel voltages will not result in a luminance which for all levels matches the case where both columns have the same pixel voltage. This is a consequence of S-shaped transmission-voltage characteristics which are typical of all twisted nematic mode LCDs. Having a constant offset voltage which is independent of the input pixel data also causes problems with fine image patterns. A checkerboard or alternating-column kind of image pattern will not be properly rendered. For certain patterns in which pixel data correspond to the offset voltage, the pattern could either be twice as intense or may disappear altogether.
Other techniques to improve the viewing angle characteristics of liquid crystal displays involve altered or special pixel structures, liquid crystal modes, or wiring within the panel array. Examples of other techniques include dual-domain TN-mode, multidomain vertical alignment (MVA) and in-plane switching (IPS). These techniques which require special structures within the glass panel are inherently more expensive to develop and manufacture than techniques which avoid special structures. The IPS mode generally requires more power in operation than the other modes. As such, these techniques have more general applicability to desktop monitors than to notebook computer displays. Furthermore, many of these approaches are generally not extendible to high density pixel arrays because special pixel structures require that a large fraction of the total available area be devoted to the purpose of viewing angle improvement. The remaining fraction limits the aperture area which can be achieved in a design as the pixel area is decreased. Complicated pixel structures are also difficult to manufacture with high yield.
Thus, there remains a need in the art to provide an efficient and low cost mechanism that improves the viewing angle characteristics of modern liquid crystal display panels, especially for notebook computer displays.
The method and apparatus of the present invention provide a very low-cost way to improve the viewing angle characteristics of liquid crystal displays. The present invention provides an efficient mechanism to modify the intensity values (in digital form) of the subpixels of the display using dithering techniques that take into consideration the non-ideal luminance characteristics of the subpixels of the panel, thereby improving the displayed image by suppressing or eliminating level reversal and color shift over a wide range of viewing angles.
According to the present invention, the data which is supplied to the panel is altered; therefore, it is not necessary to alter or change the liquid crystal cell, pixel structure, or glass panel, which are expensive and difficult to implement. The present invention can be implemented within the display subsystem, the data processing portion of the controller electronics within the display module, or operating system or application software. As the pixel density increases, the image quality and overall performance of this technique improves. Unlike other techniques which involve changes in physical pixel structure, this invention is easy to implement as the pixel density increases. This technique does not require special structures within the glass panel, and is intended for use with LCD""s with full grayscale capability, and takes full advantage of that capability, as well as the full dynamic range of the panel. In addition, image data containing text, line art, or other information can be preserved, as described in more detail below. Because only the data is altered, the method or apparatus can be controlled by the user, with the option of turning it off completely or altering the degree to which the viewing angle characteristics are changed. In this invention, both the luminance and color changes with viewing angle are reduced.
The present invention not only improves viewing angle characteristics, it can also be used to improve color management and control by restricting the subpixel colors to a range having well-behaved states, without reduction in the number of renderable colors.
This technique could be applied to any liquid crystal display which has viewing angle variations. Examples include thin-film-transistor liquid-crystal displays (TFTLCDs), otherwise known as active-matrix liquid-crystal displays (AMLCDs). The active thin film transistor devices which address the pixels in the array could be made of any material, such as amorphous silicon (a-Si), polycrystalline silicon (poly-Si), single-crystal silicon, or organic materials. The invention is also applicable to other kinds of liquid crystal display devices, such as passive-matrix LCDs, otherwise known as super-twisted nematic liquid crystal displays (STNLCDs), and ferroelectric LCDs.
In the method of generating an improved image according to the present invention, intensity values associated with the data elements of an image are modified to reduce the number of mid-tone intensity values between the bright and dark intensity values. Intensity values are modified according to the dependence of subpixel luminance on intensity and at least one viewing angle of the liquid crystal display. Intensity values are also modified according to other defined conditions on the data elements of the image. For example, if the data elements of a portion the image meet certain criteria, there is no modification of the intensity values.
In a preferred embodiment, a first plurality of entries providing an association between intensity value and luminance value for subpixels of an LCD display in at least one viewing angle direction are provided. In addition, a second plurality of entries providing an association between a target intensity value and intensity values outside the mid-tone intensity range are provided. The intensity values are modified to reduce the number of mid-tone values by: generating a first luminance value from subpixel intensity values using the first plurality of entries for image data, identifying a target intensity corresponding to that luminance by using the first plurality of entries, and identifying intensities outside the mid-tone range by using the second plurality of entries.
The preferred apparatus according to the present invention is a pixel data processor within the electronics of the display controller, implemented as part of an application-specific integrated circuit (ASIC) contained within the display panel module. The pixel data processor modifies intensity values associated with the data elements of an image to reduce the number of mid-tone intensity values between the bright and dark intensity values. Intensity values are modified according to the dependence of subpixel luminance on intensity and at least one viewing angle or range of viewing angles of the liquid crystal display.
The above and other features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.